stm32mp1: add a function to check boot is from serial device

This function will return true if the platform has booted from UART or USB,
false if it booted from a storage device.

Change-Id: I427f87cb3005c337b878430d2d484fb43236d5ea
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/188655


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Sebastien PASDELOUP <sebastien.pasdeloup-ext@st.com>
Tested-by: Sebastien PASDELOUP <sebastien.pasdeloup-ext@st.com>